Conventional bags which are adapted to be filled with fertilizers, feed, chemicals, and the like (i.e., the so-called "heavy-load bags") are typically made from polyethylene resin due to its physical strength and low cost (particularly when such bags are formed from a tubular polyethylene film by inflation molding techniques).
However, when heavy-load bags made of a polyethylene resin are filled with high temperature materials (e.g. between about 80 to 100.degree. C.) and placed one on top of the other, thermal adhesion between bags in contact with one another usually occurs. This thermal adhesion (known as "blocking" in art parlance) not only makes the bags more difficult to handle, but also increases the possibility of bag breakage. For this reason, high temperature materials are usually either cooled below about 60.degree. C before the heavy-load bags are filled, or the filled bags are cooled before being placed upon a pallet.
Examples of thermoplastic resins which are known to exhibit excellent heat resistance properties include polyethylene terephthalate resins, polyamide resins, polyphenylene sulfide resins, and polycarbonate resins. However, these resins are impractical to use for heavy-load bags due to the resins' high costs, difficult film-forming properties (particularly by inflation molding), and poor productivity during bag formation. Although blending such a heat-resistant resin with polyethylene resin has also been considered as an expedient, such a proposal has not been put to practical use due to the wide variance in mechanical and thermal properties that occur depending upon the dispersion and kneading conditions employed to form these resin blends.
Therefore, it is an object of the present invention to provide a bag having excellent blocking resistance which would thus be especially well suited for holding high-temperature materials. According to the invention, it has been found that blocking resistance for a bag can be increased by 10.degree. to 30.degree. C. when the bag includes an extruded external film layer comprised of between about 5 to 30% by weight (based on the total weight of a composition) of a particular polyethylene resin (to be described below).
The term "blocking resistance" used herein and in the accompanying claims is intended to mean a value obtained by measurement according to JIS Z 1514, items 6, 7 (incorporated by reference herein).
Accordingly, the present invention provides a bag having excellent blocking resistance characterized by including an external film layer comprised of a resin composition composed of a polyethylene (PE) resin and between about 5 to 30% by weight (based on the resin composition) of a polybutylene terephthalate (PBT) resin having an intrinsic viscosity of 0.7 or more.